CMP is used in semiconductor manufacturing for controlled metal removal and for planarization of layer surfaces within a wafer. In typical CMP processes, a polishing slurry is directed onto a rotating polishing pad. A rotating polishing head holding the wafer is moved across the polishing pad. As the process proceeds, the chemical content of the slurry and the mechanical movement of the pad combine to “polish” the wafer surface. Particles suspended in some slurries add to material removal rate. CMP is intended to remove material to isolate interconnects, reduce surface topography and render the surface planar with respect to other surfaces embedded in the wafer.
The three most widely used types of CMP polishing slurries currently are: an abrasive-containing slurry; a reactive liquid abrasive-less (RL) slurry; and a pad containing particle solutions used in fixed abrasive (“FA”) and bonded abrasive (“BA”) technologies. Two-component slurries consist of a particulate material and a chemical compound, with the particulate material constituting from about 2% to 20% by weight of the total slurry. RL slurries contain no abrasive solid or particulate matter, and hence rely on chemical reaction of the slurry for polishing action. In the third type of CMP, the particles in the pad interact with a chemical solution and the wafer to release the particles from the pad at the point of use.
Because each process type has drawbacks, much work has been directed to developing advanced solutions to improve the CMP process and particularly for copper. The success criteria for an advanced CMP slurry designed for low dielectric constant copper technology are extremely demanding. The success criteria relate to: residual metal, planarity, defectivity, manufacturability/compatibility, and electrical yield. All criteria summarized below must be met for any new CMP chemistry to compete with the traditional CMP chemistries employed in semiconductor manufacture.
“Residual” refers to the amount of material, for example metal, remaining on undesired areas such as dielectric between metal lines, after a CMP polishing or “clearing” step is completed. Ideally, no unwanted metal is left. The primary purpose of copper CMP is to isolate metal lines by polishing the wafer back to the dielectric interface. Traditional solid-based slurries accomplish this by polishing until the interface with the next material is exposed; and then continuing the polishing for some additional time. This additional polish, called “overpolish” is needed to compensate for non-uniform film removal across the wafer during CMP, or non-uniform film thickness on the wafer prior to the CMP step. For a typical multilayer integrated circuit, a whole range of features need to be cleared, from very large isolated features to very dense minimum pitch arrays. Typically, dense minimum pitch features are the last to clear, both because they plate more thickly and polish more slowly. RL slurries have shown limitations in their ability to clear dense minimum pitch features. In addition, as integrated circuits build vertically into multilayer circuit builds (MLBs), lower level topography creates enormous challenges for polishing at higher levels because of the additive effect of topography from one level to the next. RL solutions are not well-adapted to the unique polishing problems associated MLBs.
“Planarity” refers to minimizing dishing and erosion topographies arising in a CMP process which detract from achieving uniform and planar metal and dielectric surfaces throughout the wafer. Whatever a particular CMP chemistry's effectivity may be in achieving desired residual criteria, it must also avoid creating excessive dishing and erosion.
“Defectivity” refers to any surface abnormality including mechanically caused defects such as scratching, gouging, and general roughness of surfaces arising during CMP. The mechanical defects are usually caused by larger particulates in the slurry. Controlling such defects constitute one of the largest obstacles to developing advanced copper CMP chemistries based on FA or BA technologies. Minimizing surface roughness is important primarily to achieving a good adhesion interface with a subsequent passivation layer.
“Manufacturability/compatibility” is a set of criteria which for any CMP process and consumable is a measure of how robust from a manufacturing perspective the CMP process is; and how compatible it is for a range of integration schemes. RL solutions are challenging to apply in manufacturing lines. First, RL solutions tend to be very temperature sensitive, which requires that RL process temperatures be strictly maintained at considerable incremental cost. Also, during polishing, RL solutions experience “first wafer” effects where the first few wafers processed polish differently than the succeeding wafers in the lot. This creates wafer-to-wafer variations which reduce yield. In addition, a polishing solution must have a reasonable polishing rate (>2000 Angstroms/minute) to minimize cycle time in CMP. RL chemistries dependant on pH and chemical components have polishing rates that are as low as 500 Angstroms/minute, which is far too slow in a factory production environment.
“Electrical yield” refers to the ability of a copper CMP process to create wafers that consistently meet standard resistance and leakage tests.
The advantages of polishing with an RL vs. a particle-containing slurry typically are: greater planarity, lower defect levels, lower stress to wafer surfaces, and better selectivity. The advantages of using traditional particle-containing slurries are mainly a high material removal rate and a more robust manufacturing capability including larger process windows, ability to clear dense features on MLB, and better process control. The advantages of FA and BA technologies include improved planarity and better selectivity but at the price of a much higher defect rate than traditional or RL slurries.
Some work has occurred in lowering the solid content of CMP slurries as illustrated in U.S. Pat. No. 5,985,748. This teaching overcomes some deficiencies of a 1-step CMP process with a 2-step process in which step 1 uses a high abrasive content slurry to perform bulk removal of metal. Step 1 is terminated short of exposing an underlying ILD layer to abrasion that creates dishing and erosion conditions in the ILD. At this point step 2 is initiated by changing the slurry to one with solid content ranging from zero to about 1 wt %. The reduced solid concentration slurry allows for a greater control over the material removal rate and other CMP conditions for the remaining metal, thus resulting in better planarization.
What is still lacking, however, is a more versatile alternative to solid-less slurries and high solid content CMP slurries. Specifically a CMP slurry is needed which performs bulk copper removal far faster than RL slurries; which clears dense copper features on the wafer far more effectively than current RL slurries do; which lends itself to use in a continuous, i.e., one shurry CMP; and which yields more uniform results and greater stability over successive wafer runs.